# Useful Formulae

2 TRIANGLES and their various centres A rightangled triangleobeys the Law of Pythagoras the square of the hypotenuse the side opposite the right angle is equal to the sum of the squares of the other two sides opposite top left. a2 b2 c2 or equivalently c a2 b2 The sum of internal angles in any triangle 180,or radians. The perimeter P a b c. The area S bh absin C see top right diagram. Sine Rule 2r where r is the radius of the circumcircle. Medians connect corners to the midpoints of the opposite sides. The three medians meet at a point called the centroid. ma 2b2 c2a2 mb 2a2 c2b2 mc 2a2 b2c2 Altitudes are line segments drawn from each corner to the opposite side or its extension,meeting it at a right angle. ha hb hc The three altitudes meet at the orthocentre. a 2S a sinA b sinB c sinC 2S b c 2S 3